1. Field of the Invention
The present invention relates to an image forming apparatus capable of optimizing gradation expression of input image data, for example.
2. Description of the Related Art
In an image forming apparatus such as a copying machine and a printer using electrophotographic technology, an electrostatic latent image is formed on a photosensitive member by uniformly charging the photosensitive member by a charging roller and exposing the photosensitive member to laser light, for example, according to an image signal based on image data. The thus-formed electrostatic latent image is developed with toner at a developing portion, and the developed toner image is transferred onto a transfer material by a transfer roller. The toner image transferred onto the transfer material is fixed on the transfer material by a fixing device, and then the transfer material is discharged from the image forming apparatus.
In such an image forming apparatus, a high quality output image is reproduced by selecting among various gradation expression methods depending on the type of the image data (text/line-work, graphic, map, developing paper, photograph, printing, etc.). To stabilize the quality of an output image, the adjustment (calibration) of image formation conditions such as density correction and gradation correction is performed according to the state of the image forming apparatus by forming a predetermined pattern on an image bearing member in advance of the image formation and reading a density of the predetermined pattern.
The adjustment is performed for the purpose of calibrating minute fluctuations that are caused during the continuous use of the image forming apparatus in reproducibility of gradation and density of the gradually output image to standard and normal levels. The fluctuations in image density and gradation reproducibility includes a fluctuation due to a change in environment and a fluctuation due to temporal changes of the photosensitive member and the toner, and it is necessary to correct these fluctuations at once to integrate the output image density and the gradation reproducibility.
In the conventional method, a test pattern (test chart), which is an index for correction, is firstly printed out on a transfer material to find gradation characteristics of an output image of the image forming apparatus itself. Subsequently, the transfer material on which the test pattern is formed is placed on a reader unit, and patterns of gradation levels are read by the reader unit. After that, level values of the read-out gradations and reference values previously stored in the image forming apparatus are compared to each other, and, in the case where there is a difference, the difference is fed back to adjust the image processing conditions such as gradation correction to an optimum state by which standard level printing is enabled.
In performing such calibration, an operator prints out the test pattern on a transfer material for each of gradation expression methods provided in the image forming apparatus. After that, the work of placing (setting) the transfer material on the reader unit for reading is performed for a number of times that is the same as the number of gradation expression methods (e.g., for a number of times that is the same as the number of transfer materials on which the test patterns are printed) to perform the adjustment for each of the gradation expression methods. Therefore, the frequent calibration work may be bothersome for the operator, and the number of transfer materials used for the calibration is increased with a required time for the adjustment being increased. In this regard, Japanese Patent Application Laid-Open No. 2003-054078 discusses a method for performing adjustment of image processing conditions such as gradation correction based on test patterns of two types of gradation expression methods that are printed on one transfer material.
With the method, it is possible to reduce transfer material consumption as well as to shorten the time required for adjustment by printing the test patterns of two types of gradation expression methods on one transfer material. However, since the test patterns of different gradation expression methods are disposed adjacent to each other in a sub-scanning direction, the test pattern to be used for the adjustment is more subject to influences to be caused when the gradation expression method is changed. The influences include a memory image at the developing portion for developing the test pattern, and a photosensitive member memory image on the photosensitive member, formed when changing the gradation expression method. Since the test pattern is used as the index for the correction, once the test pattern is influenced by the fluctuation attributable to the fluctuations in the image forming apparatus and the fluctuations attributable to the changes in photosensitive member and toner, the test pattern influences the adjustment of the image processing conditions such as gradation correction. More specifically, in the case where test patterns of a plurality of gradation expression methods are printed on one transfer material, influence of a memory image caused in formation of a test pattern using one of the gradation expression methods tends to be exerted on formation of a test pattern using another one of the gradation expression methods.
In the case where the direction of a rotation axis of the photosensitive member is set as the main scanning direction, a direction orthogonal to the main scanning direction is referred to as a sub-scanning direction, which is orthogonal to a rotation axis of the transfer roller.
An memory image that may occur at the developing portion will be briefly described below with reference to FIG. 16. For easy understanding, an assumption of forming a pattern illustrated in FIG. 16 on one recording medium P is made. The length direction of the recording medium is a rotation direction of a photosensitive member 4 and corresponds to a conveyance direction (direction of the arrow) of the recording medium, and the width direction is an axial direction of each of the photosensitive member 4 and a developing cylinder provided in a developing portion 3 and corresponds to a main scanning direction by a laser. In the example illustrated in FIG. 16, a white hollow circle pattern is formed on a solid black image (uniformly black image) on the left half part in the axial direction of the photosensitive member 4 and the developing cylinder, and a black hollow circle pattern is formed on the right half part, followed by a halftone solid image (uniform image), which is formed by changing the gradation expression method.
In the developing portion 3 using electrophotographic technology, a powdery developer called toner is housed in a toner container inside the developing portion 3, and the toner is uniformly coated on the developing cylinder, so that the toner is conveyed to a nip portion between the developing cylinder and the photosensitive member by the rotation of the developing cylinder. During the conveyance of the toner by the developing cylinder, the toner is electrically charged by friction with the developing cylinder or friction between the toner particles, and an electrostatic latent image formed on the photosensitive member is developed as a toner image.
However, a part without an image is not developed by the toner even when the toner is transferred by the developing cylinder. Therefore, when the developing cylinder is rotated once more, a difference in toner electrical charge amount can sometimes occur on the developing cylinder between a part developed by the first rotation and a part not developed. In such case, a memory image at a rotation cycle of the developing cylinder is generated as illustrated in FIG. 16. As is apparent from FIG. 16, memory images of the solid black image, the white hollow circle pattern, and the black hollow circle pattern are formed, due to influence of the images that are formed previously, on the area on which the halftone solid image is uniformly formed by changing the gradation expression method. The memory images (abnormal images) may occur not only in the second lap but also in the third lap of the rotation of the developing cylinder as illustrated in FIG. 16, and may also occur even in the fourth and fifth laps.
In the case where the abnormal image is generated on the test pattern during the calibration, since cyclical irregularity occurs on the test pattern due to the influence generated by another gradation expression method, the image adjustment can be unsuccessful.
Also, the photosensitive member memory image means a vaguely remaining image of an image formed at a previous lap of the rotation of the photosensitive member that is subjected to image exposure depending on a state of the photosensitive member. The photosensitive member memory image is substantially similar to that illustrated in FIG. 16 except that the image is generated at the photosensitive member cycle, and the formed test pattern is subject to the influence of the photosensitive member cycle, thereby making it difficult to perform the image adjustment successfully.
It is possible to reduce the number of recording mediums to be used by printing test patterns of a plurality of gradation expression methods on one recording medium. However, the influence of the memory image generated due to the formation of the test pattern of one of the gradation expression methods is exerted on the test pattern of another one of the gradation expression methods and further on the adjustment of the image processing conditions such as the gradation correction.
The above example is described as a problem on one recording medium since the influence is of the pattern formed at a leading part of one recording medium. Also, in the case where a continuous pattern is formed from a leading end to a trailing end in the conveyance direction of the first recording medium, for example, a problem of a memory image similar to that described above occurs on a pattern to be formed on a second recording medium when an interval between the recording mediums is short. In this case, it is possible to alleviate the influence by widening the sheet feed interval between the first recording medium and the second recording medium for a predetermined multiple number of a peripheral length of the developing cylinder. However, an extra recording medium output time is required for the increase in sheet feed interval.